Skip to 0 minutes and 8 seconds If you really dig into the question of knowledge and truth, you come to quite a surprising conclusion - that there may not be such a thing as knowledge, at least in the empirical or contingent sphere, and it may be very hard to identify the truth. That might seem to be a rather despairing thing to say, but it isn’t. It teaches us a really important lesson. That the most important thing that we can hold before us always, is that whatever we think about things, whatever we believe, must be based on rational foundations. If there isn’t such a thing as complete knowledge, then there is at least rational belief. Bertrand
Skip to 0 minutes and 49 seconds Russell said: it’s not so much the question of what you believe but why you believe it that really matters. If you believe it on the basis of evidence, and on the basis of good reasons, then you have an entitlement to that belief, and that belief will be to that degree rational. Now think of that word, the first part of that word ‘rational’ is the word ratio. Ratio means proportion - proportion your beliefs and your actions, the things you think about other people in the world on the evidence that you have and on the reasons that support your thinking that way, and you will be a rational thinker.
Skip to 1 minute and 24 seconds A rational thinker always recognises that you might be wrong, you might have to revise your view when new evidence or stronger reasons come along. But to be a rational thinker is to be a responsible thinker. You don’t just believe any old thing because it’s convenient or because it’s ancient or because somebody else told you. You must always look for the evidence and for the reasons.
In this video the humanist philosopher AC Grayling emphasises the importance of basing our beliefs on rational foundations.